This invention relates to an off-the-road heavy duty pneumatic radial tire to be used on large type heavy duty vehicles on off-road conditions such as irregular ground or roads which are rugged by various transportation or works such as building or construction sites or mines. More particularly it relates to an off-the-road heavy duty pneumatic radial tire whose durability is greatly improved.
With hitherto used heavy duty pneumatic radial tires, for the purpose of reinforcing a tread of the tire, there is in general provided a belt consisting of a number of layers made of rubber coated steel cords surrounding a crown portion of a carcass toroidally extending between a pair of beads of the tire.
In such a first type of the prior art tires, steel cords are used for the belt layers and are of 1.times.3+9+15+1 construction. The cord strength is of the order of 165 kg/one cord.
In this case, the layers of the belt are first, second, third, . . . layers arranged in the order from a carcass side to a tread side. And among them, as illustrated in FIG. 1a, the widest second layer 5B is arranged on the first narrow layer 5A and the remaining layers 5C-5F are successively arranged on the second layer in a manner progressively reducing their widths. Therefore, imaginary lines connecting width edges of the respective layers form a V-shape in the form of a sharp wedge as shown in FIG. 1a.
Steel cords of the respective layers of the belt are at angles of about 21.degree. with an equatorial plane of the tire. (The angles are designated by additional characters R where the cords extend from lower left side to upper right side viewed from the outside of the tire, and L where the cords extend from lower right side to upper left side.)
With such a simple belt construction, the belt exhibits a very high rigidity against tensile forces acting upon the belt when inflated because the many layers are used and the cords of the laminated layers, as much as six, intersect with each other in the adjacent layers although the cord strength itself is relatively low. Moreover, the rigidity of the belt is enhanced by the fact that the entire belt layers are like a high strength plate and the V-shaped imaginary lines connecting the width edges of the respective belt layers are in the wedge shape.
On the other hand, rigidity of the tread rubber between the carcass and the belt at its width edges is much less than that of the belt subjected to the tensile forces. As a result, when the tread is subjected to a load, stress concentration takes place at tip ends of the V-shaped wedges to cause great shearing strains in the tread rubber which would cause troubles of the tire such as cracks.
Since the first layer of the belt is narrow, moreover, the amount of the rubber between the first layer of the belt at its edges and the carcass is less than the amount of rubber between the second layer at its edges and the carcass. Therefore, when the tread is subjected to a load to deform together with the belt, the edges of the first belt layer are subjected to a much larger tensile stress to cause a large shearing stress acting upon the rubber between the edges of the first layer and the carcass facing to the edges. This results in separations and cracks between the edges of the first belt layer and the rubber and growth of the separations and cracks of the rubber along the outer surface of the carcass facing to the edges of the first belt layer.
In contrast herewith, in a second type of prior art tire illustrated in FIG. 1b, second and third belt layers 5B and 5C among first to fifth belt layers 5A-5E arranged in the order from a carcass to a tread side form main intersecting belt layers. All of the remaining belt layers are auxiliary layers as shown in FIG. 1b.
In this case, the main intersecting belt layers are arranged such that their steel cords intersect with each other and are inclined in opposite directions relative to an equatorial plane of the tire. Circumferential tensile forces caused in a tread of the tire are supported by the cords of the main intersecting belt layers.
The fourth and fifth layers 5D and 5E as the auxiliary layers are made of so-called "high elongation cords" having breaking tensile elongations more than 1.4 times those of steel cords of the other layers. On the other hand, the first layer 5A as the auxiliary layer is made of a pair of belt layers spaced apart from each other so that the first layer 5A does not exist at the center of the carcass.
Moreover, angles of steel cords of the respective layers of the belt with an equatorial plane of the tire are for example R 62.degree., R 40.degree., L 25.degree., R 25.degree. and L 25.degree. in the order from the carcass of the tread side. The cords of the main intersecting belt layers 5B and 5C are of 1.times.3+9+15+1 construction. The cord strength is of the order of 465 kg/one cord. The cords of the first layer of the belt are of 1.times.3+9+15+1 construction and cord strength of that layer is of the order of 280 kg/one cord.
With the second type of the prior art, imaginary lines connecting width edges of the layers except the fourth and fifth layers using the high elongation cords form a V-shaped wedge so that stress concentrations would occur in the same manner as with the first type of the prior art. Particularly, when the tread is subjected to a great force such as in the case that the tire rides over stones on a road, the main intersecting layers must support excessive tensile forces because the main intersecting layers are inherently two layers. Moreover, edges of the fourth layer made of the high elongation cords do not properly cover edges of the second layer so that the stress mitigating function is insufficient. Therefore, the tire of the second type is also prone to separations at the edges of the second layer.
In this case, moreover, as the main intersecting belt layers are only two layers, the first layer of the belt having a narrow width is obliged to support excessive tensile forces so that fatigue at the edges of the first layer is prematurely promoted resulting into trouble such as separations and cracks even in an initial period of use. As above described, moreover, since the first layer is narrower than the second layer, great shearing stresses act between the edges of the first layer and the carcass to cause troubles such as separations outside of the carcass as already explained.
In a third type of the prior art as illustrate in FIG. 1c, among first to sixth layers arranged in the order from a carcass to a tread side, the first, second, third and fourth layers 5A, 5B, 5C and 5D form main intersecting belts and the remaining fifth and sixth layers 5E and 5F are made of high elongation cords. The nature of the third type as a whole is intermediate between those of the first and second types above described.
Steel cords of the respective layers are alternately arranged at angles of R 23.degree. and L 23.degree. from a carcass to a tread side. All steel cords of the main intersecting belts are of 1.times.3+9+15+1 construction and cord strength is of the order of 165 kg/one cord.
With this arrangement of the tire of the third type, rigidity of the main intersecting belts is higher than that of the second type, although it is less than that of the first type. Imaginary lines connecting width edges of the respective layers form a V-shaped wedge in the same manner as in the first and second types. Therefore, when the tread is subjected to a great force such as in the case that the tire rides over stones on a road, the stress mitigating function of the fifth layer is insufficient because the fifth layer does not sufficiently cover edges of the second layer so that troubles such as separations and cracks are likely to occur. Moreover, there is a tendency for separations and cracks to occur resulting from separations at edges of the first layer.